This invention relates to a monitor for particles of various materials which counts, on in-line and real-time basis, particles of various materials present in a variety of liquids which are used in a wet process in the course of a fabrication process of semiconductor devices.
As is well known, particles of various materials (various kinds of substances usually called dusts) cause contamination or the like of semiconductor, for example, irrespective of the fact that they are contained either in the liquids for the semiconductor fabrication process or in water or liquid chemicals for general use. Accordingly, many different makers have hitherto been engaged in developing and commercializing methods and apparatus for inspecting and counting particles of various materials present in liquids.
Many apparatus for inspection of particles have been available in the past and they are based on a variety of fundamental principles depending on utilization purposes. To sum up the principles on which the conventional apparatus of this type are based, there are three major categories of (1) an optical method, (2) an electrical method and (3) an ultrasonic method.
The ultrasonic method applies, as disclosed in for example JP-A-57-19653, techniques of a so-called sonar and inspection of a crack using an ultrasonic wave to the counting of particles in liquid but it suffers from poor reflection of the ultrasonic wave, especially, where the particles are biological cells and can not always be suitable for general purposes.
The electrical method detects, as disclosed in for example JP-A-58-53738, an increase in electrical resistance of a liquid due to replacement of part of the liquid in a pinhole with particles which pass through the pinhole. Originally, this method has been developed for counting red blood corpuscles in blood. Since the red blood corpuscles have each the size which is substantially constant, the pinhole will not be clogged if its size is larger than that of the red blood corpuscle. However, particles in the liquids used for the semiconductor fabrication process are different in size case by case with the result that the pinhole through which the particles to be counted pass may sometimes be clogged with the particles. This accounts for the fact that the electrical method is inconvenient for ordinary use in the semiconductor fabrication process involved with different sizes of particles and is not always suitable for general purposes as in the case of the ultrasonic method.
In conclusion, it is the first method, i.e., the optical method that has general compatibility with inspection for particles of various materials and can conveniently or safely be used for counting particles of unknown size. The optical method can be particularized by some family processes, of which two are closely related to the present invention and will be described below.
The noticeable first process originating from the optical method is for detecting scattering light from a particle, as will be taught from the disclosure of, for example, JP-A-51-136475.
An apparatus utilizing the scattering light is constructed in principle as shown in FIG. 6.
Referring to FIG. 6, a liquid 2 to be inspected is poured, together with particles 1, 1' (represented by black circular dots in the figure), into a special vessel 3. The liquid 2 is pressurized by a suitable device (not shown) to run past a nozzle 3a, turning into a jet 2'. The jet 2' enters a receptacle 4 and is usually discharged therefrom for disposal. For example, the jet 2' has a diameter of several of hundreds of microns and when the number of particles present in the liquid is relatively small (for example, as in the case of clean water most frequently used for the semiconductor fabrication process), the particles 1, 1' are aligned in the line and run off in jet together with the liquid 2, as best seen in FIG. 6. Accordingly, when the jet 2' is illuminated with a light beam 5 orthogonal to the jet 2', the light beam 5 illuminates the particles 1 one by one. For formation of the light beam 5, beams of light 5' from a light source such as a laser are converged by means of a lens 6.
When the particle 1 is illuminated with the light beam 5, two different phenomena take place as illustrated in FIG. 6. In the first phenomenon, photons constituting the light beam 5 are scattered at the surface of the particle 1, radiating a so-called beam of scattering light 7 in a direction different from the travelling direction of the light beam 5. The second phenomenon is that the light beam 5 is prevented from travelling, that is, stopped by the particle 1 and is not allowed to transmit to the righthand side of the jet 2'.
The aforementioned scattering light beam 7 can persist in the presence of the particle 1 and therefore collapses as the particle 1 runs off toward the receptacle 4. Accordingly, passage of the particle 1 can be inspected by detecting the scattering light beam 7 by means of a photodetector 8.
Intensity of the scattering light from the particle 1 obtained in the manner described above is converted into an electrical signal as exemplified in FIG. 7A. In the example of FIG. 7A, three particles 1 are detected. Once the intensity of the scattering light is converted into electrical pulses, the number of pulses can be counted using a well known technique of electrical signal processing to count the number of particles 1 present in the liquid. By using the known volume of the liquid 2, the number of particles per unit volume can eventually be known quantitatively.
The second process originating from the optical method is called a light extinction process wherein transparent light is detected. The principle of this second process will be described by making reference to FIG. 6 again.
On the assumption that no particle 1 is present in the jet 2', the absence of an obstacle allows the incident light beam 5 to transmit through the jet 2', thus forming a transparent light beam 5" which is detected by means of a photodetector 9. Intensity of the transparent light beam 5" detected by the photodetector 9 is decreased with the existence of the particle 1, indicating that the transparent light beam 5" extinguishes. Thus, as shown in FIG. 7B, the output waveform from the photodetector 9 changes decreasingly in one-to-one correspondence to the occurrence of the scattering light beam. As an example, the incident light beam 5 interferes with three particles in FIG. 7B. Obviously, the transparent light signal is equivalent to the scattering light signal shown in FIG. 7A and as is clear from the foregoing description, the number of particles in the liquid 2 can also be determined from the transparent light signal.
FIG. 8A illustrates another prior art example for counting particles by utilizing transmission light, which is applied, as disclosed in JP-A-50-11290, to a vessel for sedimentation adapted to remove particles from water mixed with particles. The optical method explained with reference to FIG. 6 is for counting on the so-called off-line basis wherein a small amount of liquid to be counted is averted from a real process so as to undergo sampling inspection and inevitably, it disadvantageously fails to perform real-time counting. Contrary to this, the process shown in FIG. 8A permits real-time counting.
Referring to FIG. 8A, a vessel for sedimentation 10 contains a liquid 2 in which particles 1, 1' are present. As the time elapses, the particle 1' initially present in an upper portion of the vessel 10 drops for sedimentation toward the bottom of the vessel 10.
Since the vessel 10 is typically made of an opaque material such as plastics or metal, an external light beam can not be admitted to the interior of the vessel 10. Accordingly, the side walls of the vessel 10 are partly bored and holes in the side walls are packed with a transparent material such as acrylic resin to form windows 11 and 11'. With this construction, a light beam 5 can transfer through the liquid 2 in the vessel 10. If the light beam 5 interferes with the particle 1, intensity of a transparent light beam 5" decreases so that a photodetector 9 can produce an output signal which changes decreasingly in accordance with the presence of the particle as illustrated in FIG. 8B and the number of particles can be counted based on the principle set forth so far. Light 5' from a light source (not shown) is transmitted to the neighborhood of the vessel 10 through an optical fiber 12 and converted by a lens 6 into the parallel light beam 5.
The prior art methods and apparatus for counting particles in liquid, though filling the pertinent roles in the industrial fields, have some disadvantages when intended to be applied to the semiconductor fabrication process.
The first disadvantage results from the fact that the prior art system fails to perform the real-time counting in applications to the semiconductor fabrication process. Taking the scattering light process or the light extinction process explained with reference to FIG. 6, for instance, a small amount of liquid 2 is sampled and pured into the vessel of the counting apparatus in order for the particles in the liquid to be counted. During the counting, the number of particles present in the liquid being in use for the fabrication process changes with time and at the phase of completion of particle counting, the number of particles present in the liquid is different from that of the particles sampled and counted. The light extinction process shown in FIG. 8A permits the real-time counting but it is exclusively and effectively applicable to the sedimentation phenomenon, and can not be used for the semiconductor fabrication process involved with stirred liquid because the same particle is counted many times. In any case, the prior art system sequentially discriminates the particles 1 one by one to produce a corresponding electrical pulse and in principle, it is obviously unsuited for the real-time counting.
In the second place, the prior art system disadvantageously fails to perform "in situ" counting and this defect is closely related to the first disadvantage. For example, in FIG. 8A, the concentration of the particles is greater in the neighborhood of the bottom of the vessel 10 than in the neighborhood of the upper portion of the vessel 10. Eventually, the particles 1 hardly stay in the neighborhood of the upper portion of the vessel 10 and the upper part of the liquid 2 in the vessel becomes relatively clean. Therefore, with the aim of counting particles 1 near the bottom of the vessel 10, another transmission light detector resembling the detector shown in FIG. 8A must be additionally provided near the bottom of the vessel 10. This requires that the vessel for sedimentation 10 be additionally machined, resulting in considerable industrial unprofitableness. It follows therefore that as far as the prior art system is concerned, any "in situ" counting of desired portions of the vessel is practically impossible. When making an attempt to inspect the liquid sampled at desired portions by using the processes shown in FIG. 6, many samples of liquid must be prepared in relation to lapse of time because the state of the liquid mixed with particles under sedimentation changes with time and inspection results can be obtained only through time-consuming labors, proving that such an attempt is impractical.